That's intellectually dishonest. It's like saying wire transfers or card payments are only valid after interbank settlements are finalized.
Bitcoin Lightning is cryptographically designed to be valid even if it's not yet settled on the main layer. It provides cryptographically sound mechanisms to overrule anyone that tries to "cheat". There is no mathematical way to cancel or double spend it, just like your dollars are valid when the transaction is committed in your bank's database although the money still technically hasn't left the other bank.
There are plenty of failure modes where you can lose your funds even if your wallet keys are not compromised. Try running a lightning node, make transactions worth more than a few hundred dollars and then leave your node offline for a few days. Or even more simply: ask yourself what happens to your funds if the disk on your lightning node goes bust and you don't have a recent backup.